Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond, giving them the general molecular formula \(\text{C}_n\text{H}_{2n}\). The systematic naming of these compounds is managed by the International Union of Pure and Applied Chemistry (IUPAC), which provides a standardized nomenclature system. This uniform chemical language is necessary to uniquely identify every chemical structure. Naming an alkene involves a sequence of steps that prioritize the double bond, ensuring its location and the molecule’s overall structure are precisely communicated.
Selecting the Parent Chain and Locating the Double Bond
The first step in naming an alkene is to identify the parent chain, which must be the longest continuous carbon chain that specifically includes both carbon atoms of the double bond. This rule takes precedence over simply finding the longest chain in the entire molecule. Once the parent chain is identified, the suffix of the corresponding alkane name, ‘-ane’, is replaced with ‘-ene’ to indicate the presence of the double bond. For example, a six-carbon chain containing a double bond is named as a hexene.
The carbon atoms in this parent chain must be numbered to assign the lowest possible locant to the double bond. Numbering must begin from the end of the chain that accomplishes this, giving the double bond priority over any attached substituents. The locant number used is that of the first carbon atom in the double bond. For instance, if the double bond is positioned between carbon 2 and carbon 3, the locant used is 2.
The current IUPAC recommendation places this locant immediately before the ‘-ene’ suffix, resulting in a name structure like but-2-ene instead of 2-butene. This positioning clearly separates the locant for the functional group from any locants used for substituents.
If the molecule contains multiple double bonds, the nomenclature adapts by using prefixes such as ‘-diene’ for two double bonds, or ‘-triene’ for three, while also adding an ‘a’ to the parent alkane name. For example, a hexane becomes a hexadiene. Each double bond must receive its own locant number, and these numbers are separated by commas and placed before the ‘-diene’ or ‘-triene’ suffix. This ensures that the position of every site of unsaturation is unambiguously defined.
Naming and Integrating Substituents
After the parent chain and the double bond locations have been established, the next task is to identify and name any alkyl groups or halogen atoms attached as side chains, known as substituents. These substituents are named using the same rules as for alkanes. Their positions are determined by the numbering of the parent chain, which was already set to give the double bond the lowest possible numbers.
In the final construction of the IUPAC name, the substituents are listed as prefixes before the parent chain name. Multiple identical substituents are indicated using numerical prefixes such as ‘di-‘ for two, ‘tri-‘ for three, and ‘tetra-‘ for four. These numerical prefixes are not considered when determining the alphabetical order of the substituents in the name. For example, a ‘dimethyl’ group is ordered under ‘m’ for methyl, not ‘d’ for di-.
The complete IUPAC name is assembled with the substituents listed alphabetically, each preceded by its locant number. Locants are separated from the substituent names by hyphens, and multiple locants for the same type of substituent are separated by commas. The substituent section is followed by the parent chain name, including the double bond locant and the ‘-ene’ suffix, resulting in a structure like 4-ethyl-3-methylhex-2-ene.
Designating Geometric Isomerism
Geometric isomerism describes the fixed spatial arrangement of groups around the double bond. Rotation around a double bond is severely restricted because of the lateral overlap of the \(\text{p}\) orbitals, which locks the attached groups into specific positions. This restriction leads to the existence of stereoisomers that have the same chemical formula and connectivity but different three-dimensional shapes.
For simple alkenes that have only two non-hydrogen groups attached to the double bond carbons, the traditional cis and trans system can be used. The cis configuration indicates that the two identical or higher-priority groups are located on the same side of the double bond plane. Conversely, the trans configuration means these groups are on opposite sides.
For more complex alkenes where the double bond carbons are attached to three or four different groups, the rigorous E/Z system is required. This system uses the Cahn-Ingold-Prelog (CIP) priority rules, which assign a priority to each group based on the atomic number of the atom directly bonded to the double bond carbon. The group with the higher atomic number receives higher priority.
The E/Z designation is determined by comparing the two higher-priority groups, one on each carbon of the double bond. If these two high-priority groups are on the same side of the double bond, the configuration is labeled Z, from the German word Zusammen (“together”). If the two high-priority groups are on opposite sides, the configuration is labeled E, from the German word Entgegen (“opposite”). This E or Z designation, enclosed in parentheses, is placed at the beginning of the full IUPAC name.